Thermionic discharge device control



Patented May 29, 1.79551' THERMIONIC DISCHARGE DEVICE i CONTROL Philip Godley, 2nd, Lexington, Mass., assignor to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Application March 25, 1948, Serial No. 16,952

' 2 Claims.

This invention relates to the operation of thermionic discharge devices and in particular to a method and circuit for providing constant power output from such devices, for example, from a diode. The invention is particularly adapted for use in the control of electron bombardment furnaces designed for thermally evaporating metals or similar materials in deposition processes, although it is equally adapted for use whenever a constant power output is desired from a vacuum tube.

Electron bombardment furnaces have been used in the thermal evaporation and deposition of metals but a number of limitations surround their use, among which is the necessity heretofore of operating at exceedingly low pressures, less than 0.01 micron, because of the tendency of the energy producing electrons to ionize gaseous material present in the evaporation chamber followed by destructive discharges. By keeping the pressures exceedingly low, the amount of ionization can be held Well below that which would initiate these discharges. Several examples of such electron bombardment heating of metals for vapor deposition coating processes are shown on pages 87-89 of Yarwoods High Vacuum Technique, 2nd Edition, published 1945 by Chapman 8: Hall, Ltd., and in Ruhle U. S. Patent No. 2,423,729.

A primary object of the present invention, in

its specific aspect, is to provide an electron bombardment furnace which may be successfully operated at materially higher pressures than those heretofore believed to be possible.

This is accomplished in accordance with the present invention by the provision of a control circuit which operates to extinguish even at these relatively higher pressures, discharges before they become destructive.

When the power control circuit of this invention is used in connection with furnaces of the type heretofore known, I have found that they may be operated without destructive discharge at pressures as high as 0.1 micron, thereby greatly simplifying the kind of outgassing equipment required over that requisite for maintaining the usual less than 0.01 micron pressure. But when the control circuit of this invention is utilized in association with the new and improved type of electron bombardment furnace described and claimed in a co-pending application of Brown, Serial No. 16,957 filed simultaneously herewith, the combined advantages of the Brown furnace and the present circuit permit satisfactory deposition at pressures running as high as 0.5 micron, or In some cases, even higher.

'Ihe power control circuit of this invention will therefore be described in connection with the operation of a furnace of the Brown type and for this purpose the accompanying drawings illustrate the furnace which is the subject matter of the above referred to Brown co-pending application. In the drawings, Fig. 1 is a cross-sectional view taken through such an electron bombardment furnace; and Fig. 2 is a representation of the circuit of this invention adapted for use in the operation of the furnace of Fig. 1.

As described in the above referred to co-pending application of Brown, the furnace comprises a vacuum chamber 8 in which the evaporation is to take place. In the bottom of the chamber supported on a cross-frame I is a cylindrical wall I2 defining a furnace chamber I4, with the top of the furnace chamber comprising a conical wall I 5 having a central circular opening I0 communicating with the vacuum chamber 8. Suspended in this circular opening is a drawn tantalum cup I8 reenforced by a bottom lining in the form of a tungsten plate resting on the bottom of the cup.

The cup is supported by a conical element 22 aillxed to a balance lever 24 supported on a knifeedged bar 26 mounted in blocks 28 carried by shielded insulators 30. A counterweight 32 adjustable longitudinally of the balance lever 24 acts to depress an electrical contact 34 under certain conditions as hereinafter described to make electrical connection with a cooperating contact 36 carried by insulator 38. A stop 39 limits the downward movement of cup IB-so that it never touches wall I5. The tantalum cup I8 supports a refractory crucible 40 adapted to contain a metal to be evaporated. The metal in the crucible 40 may be replenished by feeding the metal periodically into the crucible through a guide tube 42. As described in the above-mentioned Brown application, the feed of metal to the crucible 40 is preferably controlled by closing of the electrical contacts 34, 36. When these contacts are engaged the metal is fed to the crucible. When the crucible is filled with a sufficient quantity of metal the weight of this metal opens the contacts 34, 36A and the feed of metal ceases. A similar arrangement for controlling metal feed to an electron bombardment furnace in a vapor deposition coating device is shown in U. S. Patent No. 2,423,729 to Ruhle.

A thermionic emitter or cathode in the form of a filament is mounted in the furnace chamber I4 between two posts 52 and 54, the latter of which is insulated from the wall I2 by insulato! 56.

Water coils I are shown surrounding the lower portion of the furnace wall I2 for cooling purposes.

'I'he cup I8 acts as the plate in the high-voltage circuit shown in Fig. 2 of which the filament 50 is another component; and the circuit of Fig. 2 acts to provide a constant power input to the furnace.

Fig. 2 circuit includes a power supply S, a variable transformer V a constant current network, indicated at N, and a rectifier R as components of a plate circuit. In addition, a low voltage transformer for heating the filament U is connected across the output of the constant current network N and in parallel with the plate circuit.

In operation, therefore, electrons emitted by the emitter 5e bombard the cup I8 to heat the crucible 4t and its contents to vaporizing temperature by a combination of radiation and conduction.

The wall of the furnace I2 serves to confine electrons to the furnace chamber and sincethe top conical partition I5 is at ground potential and because of close clearance at Itbetween this conical partition and the cup I8 which is at high potential, there is a high field strength which diverts any electrons which attempt to pass through the opening I6 into the coating chamber to the cup I8. The advantages described in the Brown application of keeping electrons awayfrom the area just above the surface of the metal in the Crucible is thus accomplished and helps measurably to prevent destructive discharges.

If, however, despite this improved Brown construction a destructive discharge is initiated, the impedance across the filament and plate I8 will drop to an abnormally low value with a tendency to draw more current, whereupon the tuned circuit of the constant current network N will limit the current by decreasing the normal voltage. Simultaneously, because the parallelV filament circuit is connected across the output of the constant current network N, the normal voltage across the filament and the normal filament temperature will be reduced.

The reduced voltage in the plate circuit and the decreased emission resulting from the lower temperature of the filament together act to exy tinguish the destructive discharge substantially instantaneously. Meanwhile, the decreased emission will have so lowered the number of electrons emitted that, with the destructive discharge extinguished, the impedance between lament and plate is immediately restored to its original value. The restored impedance will then, in turn, immediately restore the voltages of both the plate circuit andthe filament heating circuit to their original values. In accordance with Ohms law a substantially constant power output to the furnace is thus provided, except for the very small increment of time caused by the thermal lag of the filament. The furnace, in operation, is usually brought up to temperature by gradually increasing the power input through the variable transformer V.

While the circuit of this invention is peculiarly effective in controlling electron bombardment furnace operation, it will be understood that the circuit is independently of value in other applications, for example, in hot filament ionization gauges and elsewhere; except in its speciiic aspect, the present invention is not limited to electron bombardment furnace installation.

I claim:

l. An electron bombardment furnace system for vaporizing. metals in an evacuated chamber, said system comprising a source of bombardment electrons including a cathode having a heater circuit, an electron bombardment target adjacent said cathode, said target being in heattransfer relationship to said metal to be vaporized and serving as a plate for electrons emitted from said cathode, a source of high voltage connected between said plate and cathode so that said plate is at a high positive voltage with respect to said cathode, said source of high voltage comprising a constant current network whose voltage output is a direct function of the impedance of the cathode-to-plate circuit, and a low voltage circuit for feeding a relatively low voltage to said cathode heater circuit, said low voltage circuit being connected to the output of said constant current network and supplying to said cathode heater circuit a voltage which bears a constantr ratio to the voltage supplied to said cathode-to-plate circuit, whereby any decrease in impedance in the cathode-to-plate circuit, due to a destructive ionizing discharge causes a decrease in both said h igh voltage and saidl low voltage, said voltagedecreases jointly acting to extinguish said discharge.

2. The process of' vaporizing metals in an evacuated chamber which comprises providing a source of electrons including a cathode having a heater circuit, positioning an electron bornbardment target adjacent said cathode and in heat-transfer relationship to said metal to be vaporized, said target serving as a plate for electrons emitted'from said cathode, providing a high positive voltage constant current source for said cathode-to-plate circuit with said plate positive, and tapping said constant current source to provide a low voltage for said cathode heater circuit, maintaining the cathode-to-plate current substantially constant, despite variations in the impedance of the cathode-to-plate circuit, by varying said high voltage, and maintaining said low voltage at a predetermined constant ratio to said high voltage.

rPHILIP GODLEY, 2ND.

REFERENCES CITED The following references are-ofy record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,631,626 Coolidge June v'7, 1927 1,945,751 Morrison Feb. 6, 1934 1,953,889 Mutscheller Apr. 3, 1934. 2,423,729 Ruhle July 8, 1947y 

